What is the value of the universal gas constant R in the units commonly used for the ideal gas law?

• A. 8.314 J·mol−1·K−1
• B. 0.0821 L·atm·mol−1·K−1
• C. 1.013 L·atm·mol−1·K−1
• D. 0.0821 L·bar·mol−1·K−1

Answer: (B)

R is the proportionality constant that makes PV = nRT work in the units you’re using. When pressure is in atmospheres and volume is in liters, the constant that fits those units is 0.0821 L·atm·mol−1·K−1. This value ensures that, with P in atm, V in L, n in moles, and T in kelvin, the equation yields the number of moles correctly and the units balance. If you switch to SI units (pressure in pascals and volume in cubic meters), you’d use 8.314 J·mol−1·K−1. If you use bar instead of atm, the corresponding value is about 0.08314 L·bar·mol−1·K−1.